Three-dimensional (3-D) conformal radiation therapy and intensity-modulated radiation therapy (IMRT) are used to deliver radiation precisely to a target. These advanced delivery techniques can potentially reduce the amount of normal tissue receiving high dose irradiation and thus allow for the prescription of higher doses to tumors to potentially improve the probability of local-regional disease control. However, the amount of normal tissue reduction is dependent on the amount of the treatment margin added when expanding from a clinical target volume (CTV) to a planning target volume (PTV), as required to accommodate setup variations and organ motion. This margin could potentially be reduced if the treatment volume could routinely be identified and localized with high accuracy prior to treatment delivery. The most effective approach to minimize all potential localization deviations is the use of real-time (or on-board) procedures when the patient is in the treatment position immediately prior to radiation delivery.
An array of image-guided technologies has been developed to improve localization accuracy in the treatment room. Exemplary image-guided technologies include optical, ultrasound, and radiographic imaging. Among these technologies, radiographic imaging is most frequently employed for on-board target localization.
Effective delivery of therapeutic radiation depends upon accurate localization of the therapy target at the time of treatment. In general, therapy plans are devised using the image volume from a “planning” computed tomography (CT) image set, acquired weeks in advance of the actual treatment. During setup for actual radiation therapy treatment, an attempt is made to match a patient's positioning on the treatment couch with their previous positioning during the planning CT acquisition. Historically, daily target localization was performed using external landmarks (e.g., skin markers), lasers mounted in the treatment room, and film images acquired using short exposures with the treatment beam.
Current on-board radiographic imaging techniques includes two-dimensional (2-D) (or projection) radiographic imaging using either kilovoltage (kV) or megavoltage (MV) sources, 2-D fluoroscopic kV imaging, and 3-D tomographic imaging using on-board cone-beam CT (QBCT) and CT-on-rails systems. 2-D radiographic imaging is a simple and an efficient imaging technique and delivers low radiation dose. However, verification of the target position is based primarily on visible bony structures or implanted fiducials. Although kV radiographs exhibit better tissue contrast than MV portal images, both are projection images and are sub-optimal since multi-layered 3D anatomy is projected onto a single image plane.
On-board CBCT renders 3-D anatomy and provides visualization of soft-tissue targets. The ability to match 3-D soft tissue between planning CT (or reference CT) images and on-board CBCT (or verification CBCT) images allows medical practitioners to verify true target positioning. However, CBCT application in radiation therapy for on-board targeting has several drawbacks. In one example, CBCT acquisition typically requires about 60 seconds with a full gantry rotation, which covers about 15 breathing cycles. Thus, single breath-hold CBCT is impractical. Therefore, when organs move during respiration, CBCT can only generate a blurred composite organ volume rather than a true organ volume. Although some 4-D CBCT image acquisition techniques are being investigated, they either require long imaging time (such as segmented breath-hold acquisitions), deliver poor image quality (e.g., re-binning of projection data from a gated image acquisition), or excessive dose (e.g., respiratory re-sorting technique).
In another example of a drawback of CBCT, CBCT requires full rotational clearance of the gantry with respect to the position of the patient and couch. This is potentially problematic for patients with tumors at peripheral locations (e.g., breast) or for those patient requiring substantial immobilization and supportive devices. Although smaller rotation angles (180 degrees plus a fan angle) may be reasonable for CBCT reconstruction, it still requires clearance of 360 degrees for gantry rotation.
In yet another example of a drawback of CBCT, CBCT radiation dose can range from 2 to 9 cGy for optimal image quality. Daily or weekly imaging will result in higher cumulative doses. This dose is generally applied to a much larger volume than the intended treatment volume. Reducing image dose is critical for those patients at high risk of developing second malignancies.
The above-described drawbacks can adversely affect the efficiency and efficacy of CBCT for image-guided radiation therapy (IGRT). In general, the acquisition time and clearance limitations of CBCT for on-board imaging in radiation oncology are not due to CBCT itself, but are principally due to the mechanical limitations of on-board CBCT mounted on the heavy gantry of a linear accelerator. In radiation oncology, the treatment gantry in a linear accelerator is not allowed to rotate faster than one revolution per minute (7 degrees per second limited by International Electrotechnical Commission (IEC) standards and 6 degrees per second by manufacturer specifications for safety reasons). Therefore, it is unlikely that the imaging time for a conventional treatment unit with CBCT capability will be improved upon soon. Such improvements would require regulatory changes based on safety considerations. As a result of these limitations, there exists a need for improved on-board imaging techniques that can efficiently provide 3-D anatomical information with minimal imaging dose and mechanical clearance requirements.
Digital tomosynthesis is a technique for reconstructing a stack of 3-D slices from 2-D cone-beam x-ray projection data acquired with limited source angulation (˜40°) or scan angle. By resolving overlying anatomy into slices, DTS can enhance the visibility of anatomy compared with 2-D kV or MV radiographic imaging. Furthermore, DTS requires less radiation exposure (less than 20% of CBCT dose) and unobstructed gantry rotation clearance, and can be implemented with a shorter scan time (at least a factor of 9) than CBCT. As a result of the quick nature of DTS acquisition, organ motion can likely be better managed with DTS technology. These advantages make DTS attractive for daily patient positioning verification and 3-D target localization, especially for those who may be at risk of second malignancies, such as young and pediatric patients. Because DTS uses only a small subset of CBCT projections, it will always be faster, and will have fewer clearance limitations than CBCT. Thus, the improvements available for CBCT will also likely be applicable to DTS. There will always be an incremental benefit of DTS over CBCT in terms of acquisition time and clearance. DTS will also require lower dose than CBCT for comparable image quality because the total dose is subdivided over fewer projections with DTS than CBCT, thereby mitigating the effect of additive detector noise.
Accordingly, in light of these difficulties associated with conventional radiation therapy technologies, there exists a need for improved systems and methods for radiotherapy based on DTS.